Genomic rearrangement of bovine rotavirus (NCDV strain) has been found following serial passage in cell culture at initial high multiplicity of infection. Plaque purified virus from passage 14 contained dsRNA with changes in electrophoretic pattern. Gene segment 5 could not be detected while a new band migrating between RNA segments 1 and 2 appeared. In a hybridization experiment homology was observed between gene segment 5 and the new band. Differences in the production of viral proteins between the two viruses could not be detected. Attempts to demonstrate (1) interference between these viruses and (2) genomic rearrangement in other serotypes are in progress.